1. Technical Field
The present invention relates to preload detection, and more particularly to a method for detecting a preload residual rate.
2. Description of Related Art
Conventionally, in many feeding or driving devices, such as ball screw devices, linear guides, gear mechanisms, a preload is applied between two preloaded elements that are configured to move with respect to each other during the manufacturing or fabricating process, in order to eliminate backlash and enhance structural rigidity, thereby ensuring high-speed and high-accuracy performance.
Taking a ball screw device for example, large balls are arranged between its screw and nut to provide such preload. However, during operation, the balls rub against the screw and the nut and the resultant wear tends to make the preload decrease over time, which in turn adversely affects the accuracy of positioning and eventually causes lost motion. Therefore, if an operator fails to replace the balls before the preload becomes invalid, the accuracy of the device can degrade. However, if the balls are replaced regularly without considering the wearing status, there may be usable balls replaced and wasted uneconomically. Thus, a preferable solution shall involving detecting the preload and making part replacement right before the preload reduces to a preset threshold.
A traditional approach to determining the current preload for a ball screw device is to use a force gauge hooking the nut, measuring, and converting the detected spring force into the preload of the ball screw device. However, for performing this traditional approach, the ball screw device has to be shut down because measurement during operation is impossible. Besides, the conversion involves substituting some relevant physical parameters (e.g. coefficient of elasticity), and tends to be inaccurate in the event of incorrect parameters or fault calculation. Another known preload-measuring method involves using a torque gauge to measure the ball screw's torque variation, and converting the measured torque into the screw's preload. Yet this method has its advantages related to the torque gauge that is usually costly and tends to get worn and become unusable soon. One more existing method for measuring a ball screw's preload is to use an accelerometer or a displacement meter to measure the vibration of the ball screw and estimate the remained preload accordingly. However, this method is subject to interference from noise caused by external sources of vibration, and is less accurate.